1. Field of the Invention
The present invention relates to a dispersion slope compensator for compensating chromatic dispersion and dispersion slope which are generated in signal lights having wavelengths transmitted through an optical fiber in wavelength division multiplexing (WDM) type optical communication.
2. Related Art
In an optical fiber communication system, because propagation waveform distortion caused by the wavelength dispersion (chromatic dispersion) of the optical fiber generally decreases signal quality, it is necessary to prevent the decrease in signal quality by utilizing the chromatic dispersion compensator and the like. For example, the conventional chromatic dispersion compensator includes one in which a so-called Virtually Imaged Phased Array (VIPA) is utilized (For example, see Japanese Patent Application Laid-Open (JP-A) No. 2003-294999). VIPA branches the light signal in plural light fluxes, which can spatially be identified (for example, traveling in different directions) according to the wavelengths.
The conventional VIPA type chromatic dispersion compensator will briefly be described below. FIG. 6 is a perspective view showing a configuration example of the conventional VIPA type chromatic dispersion compensator. FIG. 7 is a top view of the configuration example shown in FIG. 6.
As shown in the drawings, in the conventional VIPA type chromatic dispersion compensator, for example, after an outgoing light from an optical fiber 3 through an optical circulator 2 is converted into parallel light with a collimating lens 4, the light is focused onto one line segment with a cylindrical lens 5, and the light is incident to a space between parallel planes opposing to each other through a light entrance window 1D of a VIPA plate 1. The light incident to the VIPA plate 1 performs repeatedly multiple-reflection between a reflection multilayer film 1B and a reflection multilayer film 1C. The reflection multilayer film 1B is formed on one of planes of a substrate 1A, and the reflection multilayer film 1B has reflectance lower than 100%. The reflection multilayer film 1C is formed on the other plane, and the reflection multilayer film 1C has the reflectance of about 100%. At this point, the several-percent light is transmitted through the reflection plane and outputted to the outside of the VIPA plate 1 in each reflection on the surface of the reflection multilayer film 1B.
The lights transmitted through the VIPA plate 1 interfere with one another to form plural light fluxes having the different traveling directions according to the wavelengths. As a result, when the light fluxes are focused on one point with a lens 6, the focusing position of each light flux is moved on a straight line according to a change in wavelength. The lights, outputted from the VIPA plate 1 and focused with the lens 6, are reflected at different positions on a three-dimensional mirror 7 according to the wavelengths and returned to the VIPA plate 1 by arranging the three-dimensional mirror 7. The lights reflected from the three-dimensional mirror 7 travel in the different directions depending on the wavelength, and optical paths of the lights are shifted when the lights are returned to the VIPA plate 1. The different wavelength components propagate through different distances by changing the optical path shift amount according to the wavelengths, which performs the chromatic dispersion compensation of the input light. The three-dimensional mirror 7 is moved in parallel in a direction (X-axis direction of the drawings) perpendicular to an angular dispersion direction (Y-axis direction of the drawings) in the VIPA plate 1, and a curvature is changed in the Y-axis direction of the reflection plane of the three-dimensional mirror 7 to which the light transmitted through the lens 6 is incident. Thereby, a chromatic dispersion compensation amount becomes variable.
Thus, when a model shown in FIG. 8 is considered, behavior of the light in which the multiple-reflection is performed with the VIPA plate 1 is similar to the light in a well-known Echelon grating, which is of a step-shaped diffraction grating. Thereby, it can be considered that the VIPA plate 1 is a virtual diffraction grating. In consideration of interference conditions at the VIPA plate 1, as shown in right side of FIG. 8, upper sides of the outgoing lights interfere on the condition of a short wavelength based on an optical axis basis and lower sides interfere on the condition of a long wavelength, so that short wavelength components of the light signals having the wavelengths are outputted on the upper side and long wavelength components are outputted on the lower side. In the conventional VIPA type chromatic dispersion compensator, there are advantages that the chromatic dispersion can be compensated over a wide range, and the wavelength (transmission wavelength) of the light signal to be compensated can be changed by temperature control of the VIPA plate 1 and the like.
In a transmission system of the wavelength division multiplexing (WDM) light including the plural light signal beams having the different wavelengths, it is necessary that the chromatic dispersion compensation is appropriately performed to the light signals having the wavelengths, sometimes it is necessary to compensate a chromatic dispersion wavelength dependent property called dispersion slope. Thereby, an apparatus (hereinafter referred to as dispersion slope compensator), which can compensate the chromatic dispersion and the dispersion slope, is required.
Various configurations have been proposed in the conventional dispersion slope compensator (for example, see JP-A No. 2002-258207 and U.S. Pat. No. 6,343,866). For example, as shown in FIGS. 9 and 10, the chromatic dispersion and the dispersion slope which are imparted to light signals having the wavelengths of the WDM light can be separately controlled by providing means 8 (for example, diffraction grating and hologram) in the above VIPA type chromatic dispersion compensator. In accordance with the wavelength, the means 8 generates the optical path shift in parallel in the direction (X′-axis direction of the drawings) perpendicular to the angular dispersion direction (Y-axis direction) with respect to the lights having the wavelength to which the angular dispersion is performed with the VIPA plate 1.
However, for the conventional dispersion slope compensator, the angular dispersion of the lights having the wavelength to which the angular dispersion is performed with the VIPA plate 1 is performed in the X′-axis direction with the diffraction grating and the like. Thereby, the optical axis shift of the reflected light in the X′-axis direction is easy to occur in the three-dimensional mirror 7, which results in a problem that return light loss to the optical fiber 3 is increased.
That is, in the three-dimensional mirror 7 used for the conventional dispersion slope compensator, because the reflection plane is gradually changed from a convex surface to a concave surface with respect to the X′-axis direction, the reflection plane in the X′-axis direction has inclination with respect to the optical axis (Z′-axis of FIGS. 9 and 10) of the light focused with the lens 6. Thereby, the light is not perpendicularly incident to the reflection plane of the three-dimensional mirror 7, and the optical axis shift is generated in the reflected light. Particularly, in the case where an absolute value of the chromatic dispersion compensation amount is set larger, because the optical axis shift of the reflected light is increased, the return light loss to the optical fiber 3 is increased, or the light is not returned.
Specifically, as shown in FIG. 11, the optical axis (solid line) of an intermediate-wavelength light signal in the light signals having the plural wavelengths included in the input light becomes substantially perpendicular to the reflection plane of the three-dimensional mirror 7, so that the return light loss to the optical fiber 3 is small. On the contrary, the optical axis (broken line) of the light signal on the long wavelength side does not become perpendicular to the reflection plane of the three-dimensional mirror 7, so that the optical axis of the reflected light is largely shifted upward in FIG. 11 and the return light loss to the optical fiber 3 becomes larger.
The optical axis (alternate long and short dash line) of the light signal on the short wavelength side does not also become perpendicular the reflection plane of the three-dimensional mirror 7, so that the optical axis of the reflected light is shifted downward in FIG. 11 and the light is hardly returned to the optical fiber 3. In the sectional view of FIG. 11, although the inclination of the reflection plane of the three-dimensional mirror 7 is shown by the straight line, accurately the inclination of the reflection plane becomes a curved line corresponding to the curved shape of the reflection plane. Thereby, the optical axis shift of the reflected light corresponding to each wavelength is changed in a complicated manner by moving the three-dimensional mirror 7 parallelly in X′-axis direction.
FIG. 12 shows an example of transmission wavelength band periodically generated in the conventional dispersion slope compensator. As shown in FIG. 12, for the intermediate-wavelength light signal in the signal band of the inputted WDM light, the sufficient transmission band can be secured for the required transmittance (loss). However, for the light signals on the long and short wavelength sides, the effective transmission band cannot be secured due to the increase in loss. Thereby, the chromatic dispersion cannot be compensated.
In the conventional dispersion slope compensator, an arrangement (distance) of optical components between the three-dimensional mirror 7 and the diffraction grating and the like are not concretized. In order to decrease the increase in loss caused by the optical axis shift of the reflected light, it is necessary to optimize the optical arrangement.